Flat speaker sound/preamp the culprit?/external EQ needed?

I would also check the cartridge setup. First- how old is it and what sort of cartridge. If a moving magnet, its common to see the cantilever suspension perish after about 3-4 years. After this time the cartridge can take on either a stringent sound (suspension hardened) or a muddy sound (suspension is melting). It does not matter how long you've used it- it matters how old it is and what sort of environment its in- for example the suspension is more likely to melt if tobacco smoke is common in the environment.


If the cartridge is newer it might simply be how the cartridge is aligned in the arm, and you might also check the loading of the cartridge (if the preamp is so equipped).

You may also have a feedback issue. Turntables can be very sensitive to this and need to be well isolated. Kind of like having a microphone too close to the speakers on a PA system. However, with turntables, instead of a squeal, the feedback is in the bass region. 

Some have found that placing the turntable on a shelf extended from the wall provides a better platform than having the turntable sit on a cabinet or shelf assembly that sits on the floor. 

Thanks all. The turntable and cartridge are barely six months old. My instinct based on this is that I need a better premp. Any recommendations? This is what I have currently:
https://www.amazon.com/dp/B00IFMM7TC/ref=cm_sw_r_em_apip_tmPJ3BO59OKfL

Is the Rogue an integrated amp or a power amp?

What make/model of speakers do you have?

Where are they placed within the room?  Dimensions of the room?

Slow down we need more information. My first question is what model amp do you have with what power tubes? Is your NAD phono pre plugged into the AUX inputs or the PHONO inputs? What cartridge?